1. Field of the Invention
The invention relates to a novel precipitated silica product and a process for making the same. More specifically, the invention relates to an amorphous precipitated silica product which is useful as an additive in elastomeric formulations used for formed products, particularly rubber passenger tire treads.
2. Description of the Related Art
As disclosed in Kirk-Othmer, Encyclopedia of Chemical Technology, Third Edition, Volume 20, John Wiley Sons, New York, p. 779 (1982), precipitated silica product is used, inter alia, as a filler for rubber. In general, fillers are used as reinforcing agents to improve the mechanical properties of rubber and other elastomers.
Any filler used in elastomeric applications should be easy to handle and easy to formulate into elastomeric mixtures. In powder form, silica products can be difficult to handle because of poor flow and high dust generation. Also, the low bulk density of powder silica products impedes incorporation into elastomers.
While formed silica products can obviate these shortcomings to a certain extent, dispersion of the formed silica product in the elastomer can be difficult, and the degree of reinforcement achieved may be less than would be attained with a powdered silica product. Optimum reinforcing properties are generally achieved when the silica product is homogeneously dispersed throughout the elastomer matrix in a finely-divided state. Therefore, an ideal formed silica product should readily combine with the elastomer matrix and thereafter deagglomerate or disintegrate into a fine powder in the matrix, said fine powder being readily dispersible to a homogeneous state.
In addition, a silica filler should minimize the need for expensive coupling agents in the rubber formulation. Coupling agents are typically tri-alkoxy silanes possessing an amino, mercapto, polysulfidic or other functional group, and are used to reduce heat buildup/hysteresis and to enhance the mechanical improvements provided by the silica filler.
In the case of rubber formulations for the tire industry, and in particular solution styrene-butadiene rubber (s-SBR) for passenger tire treads, there are several desirable yet contradictory rubber tread properties which are strongly affected by the physical properties of the filler employed. The ability of the tread to adhere to the ground whether the surface is dry, wet, snow-covered or ice-covered is very important, and there are known silica fillers which can provide good tire traction under various conditions. However, known traction-enhancing silica fillers do not simultaneously provide the reduced rolling resistance, low tread wear rate and limited structural deformation also demanded of tires. Similarly, silica fillers which improve the rolling characteristics and durability of tire treads have historically done so at the expense of traction, and have typically required high loadings of expensive coupling agents to boost traction. Moreover, known highly-dispersible silica fillers for rubber fail to enhance the processability of rubber.
Consequently, there has been much effort in the silica filler/passenger tire tread formulation art to develop a highly-dispersible silica filler which provides an optimized combination of these contradictory tread properties while enhancing processability. See, e.g., U.S. Pat. Nos. 5,089,554, 5,227,425 and 5,403,570, as well as International Application Nos. WO 95/09127 and WO 95/09128. However, these efforts have at best resulted in unsatisfactory compromises between highly desirable characteristics. Thus, there remains a long-felt need in the art for a highly-dispersible silica filler which, when incorporated into rubber compounds, particularly rubber blends of solution SBR with one or more additional polymers, provides enhanced processability, low rolling resistance, high durability, high all-weather traction and reduced coupling agent demand for passenger tire treads fashioned therefrom or other rubber/elastomer dynamic applications.